[cema]

  If most people's usual driving speed is "10 mph over the limit," 
  then the effective speed limit is simply N+10 mph, 
  where N is the number on the signs.

A big "if". As I recall, when speed limits were raised on some of the highways in Maryland from 55 mph to 65 mph, the traffic kept going at 72 mph.

In a society where everybody drives, speeds are set not so much by some kind of a law but more by the natural course of human actions.

[mindcrime]

Yep, traffic is largely self-regulating. Most people know how fast it's safe to drive and do it anyway. The others don't care about the "speed limit". Net effect, the actual posted speed limit and associated laws and corrupt bureaucracy are totally useless.

And never mind that "speed limits" are reverse engineered from observations of the natural speed of traffic in a given area in the first place.[1]

[1]: http://www.ite.org/standards/speed_zoning.pdf